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SubjectRe: [PATCH] selinux: refactor mls_context_to_sid() and make it stricter
On Mon, Aug 6, 2018 at 5:19 PM Jann Horn <jannh@google.com> wrote:
>
> The intended behavior change for this patch is to reject any MLS strings
> that contain (trailing) garbage if p->mls_enabled is true.
>
> As suggested by Paul Moore, change mls_context_to_sid() so that the two
> parts of the range are extracted before the rest of the parsing. Because
> now we don't have to scan for two different separators simultaneously
> everywhere, we can actually switch to strchr() everywhere instead of the
> open-coded loops that scan for two separators at once.
>
> mls_context_to_sid() used to signal how much of the input string was parsed
> by updating `*scontext`. However, there is actually no case in which
> mls_context_to_sid() only parses a subset of the input and still returns
> a success (other than the buggy case with a second '-' in which it
> incorrectly claims to have consumed the entire string). Turn `scontext`
> into a simple pointer argument and stop redundantly checking whether the
> entire input was consumed in string_to_context_struct(). This also lets us
> remove the `scontext_len` argument from `string_to_context_struct()`.
>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> Refactored version of
> "[PATCH] selinux: stricter parsing in mls_context_to_sid()" based on
> Paul's comments. WDYT?
> I've thrown some inputs at it, and it seems to work.
>
> security/selinux/ss/mls.c | 178 ++++++++++++++-------------------
> security/selinux/ss/mls.h | 2 +-
> security/selinux/ss/services.c | 12 +--
> 3 files changed, 82 insertions(+), 110 deletions(-)

Thanks for the rework, this looks much better than what we currently
have. I have some comments/questions below ...

> diff --git a/security/selinux/ss/mls.c b/security/selinux/ss/mls.c
> index 39475fb455bc..2fe459df3c85 100644
> --- a/security/selinux/ss/mls.c
> +++ b/security/selinux/ss/mls.c
> @@ -218,9 +218,7 @@ int mls_context_isvalid(struct policydb *p, struct context *c)
> /*
> * Set the MLS fields in the security context structure
> * `context' based on the string representation in
> - * the string `*scontext'. Update `*scontext' to
> - * point to the end of the string representation of
> - * the MLS fields.
> + * the string `scontext'.
> *
> * This function modifies the string in place, inserting
> * NULL characters to terminate the MLS fields.
> @@ -235,22 +233,21 @@ int mls_context_isvalid(struct policydb *p, struct context *c)
> */
> int mls_context_to_sid(struct policydb *pol,
> char oldc,
> - char **scontext,
> + char *scontext,
> struct context *context,
> struct sidtab *s,
> u32 def_sid)
> {
> -
> - char delim;
> - char *scontextp, *p, *rngptr;
> + char *sensitivity, *cur_cat, *next_cat, *rngptr;
> struct level_datum *levdatum;
> struct cat_datum *catdatum, *rngdatum;
> - int l, rc = -EINVAL;
> + int l, rc, i;
> + char *rangep[2];
>
> if (!pol->mls_enabled) {
> - if (def_sid != SECSID_NULL && oldc)
> - *scontext += strlen(*scontext) + 1;
> - return 0;
> + if ((def_sid != SECSID_NULL && oldc) || (*scontext) == '\0')
> + return 0;
> + return -EINVAL;

Why are we simply not always return 0 in the case where MLS is not
enabled in the policy? The mls_context_to_sid() is pretty much a
no-op in this case (even more so with your pat regardless of input and
I worry that returning EINVAL here is a deviation from current
behavior which could cause problems.

> }
>
> /*
> @@ -261,113 +258,94 @@ int mls_context_to_sid(struct policydb *pol,
> struct context *defcon;
>
> if (def_sid == SECSID_NULL)
> - goto out;
> + return -EINVAL;
>
> defcon = sidtab_search(s, def_sid);
> if (!defcon)
> - goto out;
> + return -EINVAL;
>
> - rc = mls_context_cpy(context, defcon);
> - goto out;
> + return mls_context_cpy(context, defcon);
> }
>
> - /* Extract low sensitivity. */
> - scontextp = p = *scontext;
> - while (*p && *p != ':' && *p != '-')
> - p++;
> -
> - delim = *p;
> - if (delim != '\0')
> - *p++ = '\0';
> + /*
> + * If we're dealing with a range, figure out where the two parts
> + * of the range begin.
> + */
> + rangep[0] = scontext;
> + rangep[1] = strchr(scontext, '-');
> + if (rangep[1]) {
> + rangep[1][0] = '\0';
> + rangep[1]++;
> + }
>
> + /* For each part of the range: */
> for (l = 0; l < 2; l++) {
> - levdatum = hashtab_search(pol->p_levels.table, scontextp);
> - if (!levdatum) {
> - rc = -EINVAL;
> - goto out;
> - }
> + /* Split sensitivity and category set. */
> + sensitivity = rangep[l];
> + if (sensitivity == NULL)
> + break;
> + next_cat = strchr(sensitivity, ':');
> + if (next_cat)
> + *(next_cat++) = '\0';
>
> + /* Parse sensitivity. */
> + levdatum = hashtab_search(pol->p_levels.table, sensitivity);
> + if (!levdatum)
> + return -EINVAL;
> context->range.level[l].sens = levdatum->level->sens;
>
> - if (delim == ':') {
> - /* Extract category set. */
> - while (1) {
> - scontextp = p;
> - while (*p && *p != ',' && *p != '-')
> - p++;
> - delim = *p;
> - if (delim != '\0')
> - *p++ = '\0';
> -
> - /* Separate into range if exists */
> - rngptr = strchr(scontextp, '.');
> - if (rngptr != NULL) {
> - /* Remove '.' */
> - *rngptr++ = '\0';
> - }
> + /* Extract category set. */
> + while (next_cat != NULL) {
> + cur_cat = next_cat;
> + next_cat = strchr(next_cat, ',');
> + if (next_cat != NULL)
> + *(next_cat++) = '\0';
> +
> + /* Separate into range if exists */
> + rngptr = strchr(cur_cat, '.');
> + if (rngptr != NULL) {
> + /* Remove '.' */

On the chance you need to respin this patch, you can probably get rid
of the above comment and the if-body braces; we don't have "Remove X"
comments in other similar places in this function.

> + *rngptr++ = '\0';
> + }
>
> - catdatum = hashtab_search(pol->p_cats.table,
> - scontextp);
> - if (!catdatum) {
> - rc = -EINVAL;
> - goto out;
> - }
> + catdatum = hashtab_search(pol->p_cats.table, cur_cat);
> + if (!catdatum)
> + return -EINVAL;
>
> - rc = ebitmap_set_bit(&context->range.level[l].cat,
> - catdatum->value - 1, 1);
> - if (rc)
> - goto out;
> -
> - /* If range, set all categories in range */
> - if (rngptr) {
> - int i;
> -
> - rngdatum = hashtab_search(pol->p_cats.table, rngptr);
> - if (!rngdatum) {
> - rc = -EINVAL;
> - goto out;
> - }
> -
> - if (catdatum->value >= rngdatum->value) {
> - rc = -EINVAL;
> - goto out;
> - }
> -
> - for (i = catdatum->value; i < rngdatum->value; i++) {
> - rc = ebitmap_set_bit(&context->range.level[l].cat, i, 1);
> - if (rc)
> - goto out;
> - }
> - }
> + rc = ebitmap_set_bit(&context->range.level[l].cat,
> + catdatum->value - 1, 1);
> + if (rc)
> + return rc;
> +
> + /* If range, set all categories in range */
> + if (rngptr == NULL)
> + continue;
> +
> + rngdatum = hashtab_search(pol->p_cats.table, rngptr);
> + if (!rngdatum)
> + return -EINVAL;
> +
> + if (catdatum->value >= rngdatum->value)
> + return -EINVAL;
>
> - if (delim != ',')
> - break;
> + for (i = catdatum->value; i < rngdatum->value; i++) {
> + rc = ebitmap_set_bit(&context->range.level[l].cat, i, 1);
> + if (rc)
> + return rc;
> }
> }
> - if (delim == '-') {
> - /* Extract high sensitivity. */
> - scontextp = p;
> - while (*p && *p != ':')
> - p++;
> -
> - delim = *p;
> - if (delim != '\0')
> - *p++ = '\0';
> - } else
> - break;
> }
>
> - if (l == 0) {
> + /* If we didn't see a '-', the range start is also the range end. */
> + if (rangep[1] == NULL) {
> context->range.level[1].sens = context->range.level[0].sens;
> rc = ebitmap_cpy(&context->range.level[1].cat,
> &context->range.level[0].cat);
> if (rc)
> - goto out;
> + return rc;
> }
> - *scontext = ++p;
> - rc = 0;
> -out:
> - return rc;
> +
> + return 0;

In the case where we have a MLS policy loaded (pol->mls_enabled != 0)
and scontext is empty (scontext[0] = '\0'), we could end up returning
0 couldn't we? It seems like we might want a quick check for this
before we parse the low/high portions of the field into the rangep
array.

> }
>
> /*
> @@ -379,21 +357,19 @@ int mls_context_to_sid(struct policydb *pol,
> int mls_from_string(struct policydb *p, char *str, struct context *context,
> gfp_t gfp_mask)
> {
> - char *tmpstr, *freestr;
> + char *tmpstr;
> int rc;
>
> if (!p->mls_enabled)
> return -EINVAL;
>
> - /* we need freestr because mls_context_to_sid will change
> - the value of tmpstr */
> - tmpstr = freestr = kstrdup(str, gfp_mask);
> + tmpstr = kstrdup(str, gfp_mask);
> if (!tmpstr) {
> rc = -ENOMEM;
> } else {
> - rc = mls_context_to_sid(p, ':', &tmpstr, context,
> + rc = mls_context_to_sid(p, ':', tmpstr, context,
> NULL, SECSID_NULL);
> - kfree(freestr);
> + kfree(tmpstr);
> }
>
> return rc;
> diff --git a/security/selinux/ss/mls.h b/security/selinux/ss/mls.h
> index 9a3ff7af70ad..67093647576d 100644
> --- a/security/selinux/ss/mls.h
> +++ b/security/selinux/ss/mls.h
> @@ -34,7 +34,7 @@ int mls_level_isvalid(struct policydb *p, struct mls_level *l);
>
> int mls_context_to_sid(struct policydb *p,
> char oldc,
> - char **scontext,
> + char *scontext,
> struct context *context,
> struct sidtab *s,
> u32 def_sid);
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index dd2ceec06fef..9212d4dd817a 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -1367,7 +1367,6 @@ int security_sid_to_context_force(struct selinux_state *state, u32 sid,
> static int string_to_context_struct(struct policydb *pol,
> struct sidtab *sidtabp,
> char *scontext,
> - u32 scontext_len,
> struct context *ctx,
> u32 def_sid)
> {
> @@ -1428,15 +1427,12 @@ static int string_to_context_struct(struct policydb *pol,
>
> ctx->type = typdatum->value;
>
> - rc = mls_context_to_sid(pol, oldc, &p, ctx, sidtabp, def_sid);
> + rc = mls_context_to_sid(pol, oldc, p, ctx, sidtabp, def_sid);
> if (rc)
> goto out;
>
> - rc = -EINVAL;
> - if ((p - scontext) < scontext_len)
> - goto out;
> -
> /* Check the validity of the new context. */
> + rc = -EINVAL;
> if (!policydb_context_isvalid(pol, ctx))
> goto out;
> rc = 0;
> @@ -1491,7 +1487,7 @@ static int security_context_to_sid_core(struct selinux_state *state,
> policydb = &state->ss->policydb;
> sidtab = &state->ss->sidtab;
> rc = string_to_context_struct(policydb, sidtab, scontext2,
> - scontext_len, &context, def_sid);
> + &context, def_sid);
> if (rc == -EINVAL && force) {
> context.str = str;
> context.len = strlen(str) + 1;
> @@ -1959,7 +1955,7 @@ static int convert_context(u32 key,
> goto out;
>
> rc = string_to_context_struct(args->newp, NULL, s,
> - c->len, &ctx, SECSID_NULL);
> + &ctx, SECSID_NULL);
> kfree(s);
> if (!rc) {
> printk(KERN_INFO "SELinux: Context %s became valid (mapped).\n",
> --
> 2.18.0.597.ga71716f1ad-goog

--
paul moore
www.paul-moore.com

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